This invention relates to fairings for reducing drag forces on a pipe or other structural component due to relative movement of that pipe or other structural component with respect to a fluid medium in which it is immersed. The fairings disclosed herein are especially adapted to marine risers equipped with syntactic foam buoyancy modules which are used in deep water off-shore drilling.
As the search for off-shore oil has led drillers into ever-deeper water, the use of buoyant riser systems has been greatly expanded. The most widely used floatation material is the family of plastic composites called syntactic foams. Syntactic foam is any materials system comprising hollow microspheres dispersed in some form of binder.
Typical syntactic foams used in hydrospace applications contain fillers in the form of tiny glass bubbles, or microspheres. Because of their small size-a diameter range of 60 to 80 microns is common-microspheres appear as fine white powder to the naked eye. The microspheres are mixed into a liquid thermosetting plastic such as epoxy or polyester resin. The resulting material may have a viscosity varying from that of heavy cream to a semi-solid paste. Once, catalyzed, the resin binder hardens so that the foam becomes a rigid mass with remarkable properties: much lighter than the plastic resin, yet much more resistant to hydrostatic pressure than were the microspheres. To further enhance the quality of lightness, plastic macrospheres (up to 3" diameter) may be interspersed in the microspheres-resin mix.
In a typical installation, the riser pipe itself is suited with syntactic foam buoyancy modules. In the usual case 6 modules are used per riser pipe joint. The modules are hemi-cylinders, strapped or clamped to the riser pipe. In addition to supporting the riser, the modules may perform many other functions, such as cradling and protecting the auxiliary lines and providing fixation points for electric cables. It is possible to design the modules so they stiffen the riser, or, if the opposite condition is desired, so that maximum flexibility is assured. Provision is made to clear the choke and kill line clamps and any other fixtures on the riser. Adequate room is left at either end of the riser for make-up and running: the lower end accommodates air wrenches or other tools used when assembling joints, and the upper end clears the spider dogs.
A typical syntactic foam riser module designed to provide 98% buoyancy for an 18.625 inch (diameter) 50 foot (length) marine riser may have the following specifications:
______________________________________ Diameter 38 inches Length 15 feet Weight 1150 lbs. Buoyancy 1420 lbs. Maximum Service 3000 feet Depth Ultimate or 5000 feet Crush depth ______________________________________
Riser pipes are now being used at ocean depths of up to 5,000 feet. At these depths it is considered mandatory to use buoyancy means with the riser. Substantial water drag and vortex shedding vibration induced by currents present a great danger to riser pipes of such great length. The vortex shedding vibration causes the riser pipe to shake, which leads to fatigue, which eventually leads to deterioration of the steel in the pipe. Hollow two piece fairings of the usual shape, having ribs and made with solid plastic wall have been used for many years to protect riser pipes, including riser pipes equipped with syntactic foam buoyancy modules.
An object of the invention is to provide a fairing that has superior operation and a reduced front to rear chord length.
Another object of the invention is to provide a fairing constructed of low density material and having adequate strength for long use with repeated installation and removal.
Yet another object of this invention is to provide a fairing for a marine riser which exhibits neutral buoyancy without the addition of separate buoyancy materials.
A further object of the invention is to provide a fairing assembly that may be easily attached and removed from a marine riser.
A still further object is to provide a fairing which is compatible with syntactic foam buoyancy material.
Still another object is to provide a fairing which can be used for a broad range of risers, both buoyant and non-buoyant, since existing fairing systems are not easily adaptable to different types of risers.